There is an increasingly widespread use of servo-assisted gear changes, which are structurally similar to a manual gear change of a traditional type but for the fact that the clutch pedal and the gear lever operated by the driver are replaced by corresponding electrical or hydraulic servos. Using a manual servo-assisted gear change, the driver only has to issue the order to pass to a higher gear or else to a lower gear to a transmission control unit, and the transmission control unit autonomously carries out gear change by acting both on the engine and on the servos associated to the clutch and gear change.
An order for change of gear can be generated manually, i.e., following upon a command imparted by the driver, or else automatically, i.e., independently of the action of the driver. When the order for performing a change of gear is generated, the transmission control unit drives the clutch servo for opening the clutch so as to separate a primary shaft of the gear change mechanically from an engine shaft. At the same time, the transmission control unit acts on the engine control unit in order to reduce temporarily the driving torque supplied by the engine.
Once the transmission control unit has verified opening of the clutch, it drives the gear-change servo to disengage the gear currently engaged. When the transmission control unit has verified disengagement of the gear, it drives the gear-change servo for displacing the gear-change control shaft so as to enable engagement of the new gear. Once the transmission control unit has verified that the gear-change control shaft has reached the desired position, it drives the gear-change servo for engaging the new gear.
Finally, when the transmission control unit has verified engagement of the new gear, it drives the clutch servo for closing the clutch so as to render the primary shaft of the gear change and the engine shaft angularly fixed to one another. At the same time, the transmission control unit acts on the engine control unit for restoring the driving torque supplied by the engine.
Generally, the gear-change servo is of a hydraulic type and acts on a gear-change control shaft to impress on the control shaft both an axial displacement, i.e., along a central axis, for selecting the range of the gears, and a rotation about the central axis for engaging and disengaging the individual gears. Consequently, the gear-change servo comprises a first hydraulic actuator, mechanically coupled to the control shaft for axial displacement of the control shaft, and a second hydraulic actuator, mechanically coupled to the control shaft for rotating the control shaft.
In the first servos produced, the two hydraulic actuators were both directly coupled to the control shaft and acted on the control shaft independently of one another. However, said constructional solution is cumbersome and costly. For this reason an alternative embodiment has been proposed, in which the first hydraulic actuator is directly coupled to the control shaft for displacing the control shaft axially, whilst the second hydraulic actuator is coupled to a cam engaged by a pin fixed to the control shaft so as to render the cam angularly fixed to a fixed frame. When the cam is angularly fixed to the fixed frame, then the axial displacement of the control shaft forces the control shaft to perform a rotation as a result of the mechanical coupling between the pin and the cam, whereas, when the cam is not angularly fixed to the fixed frame, then the axial displacement of the control shaft brings about a rotation of the cam and not of the control shaft. In servos of the type described above, the second hydraulic actuator acts on a mechanical brake, which is fixed to the fixed frame and is displaced linearly in order to engage an outer surface of a dome, defined in which is the cam.
However, also the constructional solution described above is relatively cumbersome.
DE10033807 discloses a change device for a gear wheel gear change. The change device has a housing, a change shaft, a selection sleeve, brake engagement elements, a brake actuator element and an actuator element; the brake actuating element that is fixed with respect to the selection sleeve is in contact with a concentric radial annular surface of the selection sleeve and the geometric brake axis is coaxial to the central axis of the selection sleeve.